It really does depend on a large number of factors - such what you sell, where your natural audience is, where you're able or willing to ship (assuming a physical product, that is), in what language(s) you are able to write great ads, in what languages(s) your site is written, and on and on.
I'm guessing you could ask 50 advertisers this question and get 75 different answers, with most revolving around the central truth of "It depends". ;)
AWA